1. Field of the Invention
The present invention relates to a disk drive that performs perpendicular magnetic recording. More particularly, the invention relates a technique of writing sync marks in the data sectors of a disk.
2. Description of the Related Art
In most disk drives, a representative example of which is a hard disk drive, user data supplied from a host system (e.g., a personal computer) are divided into 4096-bit (512-byte) data blocks. The data blocks iare recorded in the recording region of a disk-shaped recording medium (hereinafter called “disk”). The recording region of the disk is managed in units of so-called data sectors. Each data block (i.e., part of the user data) is recorded in one data sector, together with other data.
Each data block is stored in one data sector, in a specific format. That is, the data block consists of a preamble, a sync mark, user data, and ECC (error correction code) data. The preamble is a signal of a prescribed frequency. The preamble is used to achieve AGC (auto gain control) for adjusting the amplitude of a signal reproduced from the data sector by the magnetic head (hereinafter referred to as “head”) or to accomplish clock synchronization for decoding data. The sync mark is a bit pattern that is used to detect the header of the user data.
The recent trend in the art is to divide the sync mark into two sync marks. Thus, the user data is divided into two user-data items. The first user-data item (bit length: X bits) is recorded between the first and second sync marks, and the second user-data item (bit length: 4096-X bits) is recorded, following the second sync mark. (See, for example, U.S. Pat. No. 5,844,920, 1996, and Jpn. Pat. Appln. KOKAI Publication 2001-143406.)
The first sync mark is used to detect the header of the first user-data item that follows it. It is, for example, a random pattern having a bit length of, for example, about 10 to 50 bits. In the disk drive, a decoder decodes the reproduced signal, generating a series of bits. The series of bits is compared with a reference bit pattern for the first sync mark. Namely, pattern matching is performed, thereby to detect the first sync mark.
More precisely, the first sync mark is detected when its bit pattern is found identical to the reference bit pattern. Once the first sync mark is detected, the bit that follows the last bit of the first sync mark is recognized as the first bit of the user-data item that follows the first sync mark. The decoding of the user-data item is then started. In practice, the first sync mark is considered to have been detected, even if its all bits, but two, are identical to the bits of the reference bit pattern.
The first sync mark may not be detected due to the thermal asperity (TA) that has resulted mainly from the characteristics of the GMR element that is used as the head. (For details of TA, see Jpn. Pat. Appln. KOKAI Publication 10-49806.) If the first sync mark is not detected, an attempt will be made to detect the second sync mark.
If the first sync mark is not found and the second sync mark is detected, the first data-user item (bit length: X bits) is considered erroneous data or deleted data. The first data-user item is correctly decoded, whenever necessary, through correction process that uses the ECC data.
As pointed out above, the second sync mark is used when the first sync mark cannot be detected, mainly because of thermal asperity (TA). In view of this, the second sync mark should have such a bit pattern that it may be detected at high probability in spite of the thermal asperity.
The second sync mark used in the conventional disk drive that performs longitudinal magnetic recording usually has a bit pattern that is a series of “0s” and “1s” arranged in accordance with the NRZ (non-return to zero) rule. In the longitudinal magnetic recording, any signal to be reproduced, which corresponds to such a bit pattern, has constant amplitude and can hardly be “DC-erased.”
When thermal asperity (TA) develops, the base line for signals reproduced shifts, as is confirmed in the art. As a result, any signal reproduced changes in amplitude, very likely to cause an error in the detection of data. The second sync mark is more liable to detection error than the first-sync mark, due to the shifting of the base line.
Any signal reproduced by the disk drive that performs perpendicular magnetic recording has a low-frequency component that contains a DC component. Thus, any signal reproduced undergoes a base-line shift that is called “low-band cutoff strain” when the read channel has such a transfer characteristic that it cuts off low-frequency components.
To the read channel having low-band cutoff characteristic, the second sync mark is problematical, because its bit pattern is a series of “0s” and “1s” that are in accordance with the NRZ (non-return to zero) rule. The base-line offset, which the read channel exhibits for the second sync mark that has fixed amplitude, is almost maximal at the end part of the second sync mark. Inevitably, the base-line offset persists for some time in the user data, which follows the second sync mark and which is a random bit pattern.
In short, the base-line offset caused by the second sync mark persists for some time in any disk drive that performs perpendicular magnetic recording. Consequently, the disk drive may fail to detect the first several bits of the user data, which immediately follow the second sync mark.